At the end of the G1 phase of the cell cycle, initiation of DNA replication is executed by multi-protein assemblies. This process involves the recruitment of DNA replication origins of factors such as ORC (origin replication complex), cdc6, and MCM (mini-chromosome maintenance) proteins to form a pre-replication complex (pre-RC) which dissociates at the beginning of the S phase. My research encompasses the structural and functional characterization of eukaryotic and archaebacterial MCM proteins. Previous studies demonstrate that MCM complexes display ATP-dependent helicase activity in vivo and in vitro. Eukaryotic MCM's form heterohexameric complexes consisting of six distinct subunits that are similar by sequence homology. In contrast, archaebacterial genomes contain only one MCM protein that is believed to express and purify individual MCM proteins for crystallization screening. The initial step is to elucidate the fold of a single MCM subunit using x-ray crystallography followed by the structure determination of an entire MCM assembly.